Electron lens structure



June 13, 1944. F. GRAY ELECTRON LENS STRUCTURE Filed May 3l, 1941 2 Sl'ueets-Sheerl 2 ACTUAL PATH T' SSUMED PATH 5 2 53 Num/Flea Y src/VAL /NVENTOR F GRAY A ORA/EV Patented June 13, 1944 A zssisti 2,351,501 ELECTRON LENS STRUCTURE Frank Gray, East Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 31, 19417, SerialJNo. (Cl. 315-17) 21 Claims.

This invention relates to electron discharge devices and more particularly to the reduction of aberration in electron lens systems.

, It is an object of this invention to providenovel electron lens structures. Y

. The two-dimensional analog of the three-dimensional electron lens comprising two concentric tubes, which analog may be either a pair fn where f is the focal distance for rays entering the lens parallel to the axis and at some distance from the axis, and fo is the focal distance for an electron path very near the axis. For a perfect lens the aberration ratio is equal to unity. The inverse aberration ratio is the reciprocal of the aberration ratio. The present invention, in one of its more important aspects, relates to a twodimensional electron lens structure in which the inverse aberration ratio at the edgeM of the lens is reduced from infinity (the inverse aberration ratio at the edge of the lens in the usual two-dimensional analog) to a value near that of a perfect lens.

In accordance with the invention, in one embodiment thereof, chosen by way of example to illustrate the principles of the invention there is provided an electron lens structure comprising two pairs of plates, one pair being closer to the beam generating means than the other pair and the respective plates of each pair being located `on opposite sides of the beam axis and the plates of each pair being ared outwardly in the region where they face each other. An abrupt are is required particuarly in tubes where the walls are used as sweep platesand they should therefore spaced along the axis of the tube, the potential of the plates of the middlepairbeing negative with respect to the potential of the plates ci both of the outer pairs. The potential of each of the negative plates is chosen so that the bears; is focussed at a desired point.

The invention will be more fully understood referring to the following description taken in. accordance with the accompanying drawings forming a, part thereof in which: e

Fig. 1 shows a two-dimensional analog o a spherical electron'lens systerncomprising two concentric tubes;

Fig. 2 shows a two-,dimensional electron lens system corrected for aberration;

Fig. 3 showsa combination of two corrected two-dimensional electron lenses:

Figs. 4 to 7, inclusive, are graphical representa tions to aid in explaining jthe principles of the invention; and

. Fig, 8 shows a tube in which a lens system made in accordance with the principles oi' this invention is used.

Referring more specifically to the drawings, Fig. 1 shows two pairs of plates l0, ii and l2, i3 constituting, by means of the potentials Vi and V2 applied thereto, a'two-dimensio'nal electron lens structure analogous to a cylindrical positive optical lens shown below the electron lens structure. pair are located on opposite sides of the axis of the tube and are closer to the beam generating means than are the plates i2, i3 which are also placed on opposite sides of the axis of the cathode beam. The ratio of the potentials Vi and V2 is chosen so that the cathode rays passing successively between the pairs of plates iii, il and I2,4 I3 are focussed to a line in a desired plane to the right of the pair of plates it, it. The manner in which this focussing takes place is well understood in the art and will not beV de scribed in detail in this speciiication.

In a two-dimensional electron lens such as is shown in Fig. 1, let a: be a coordinate along the axis of the lens and y be a coordinatenormel to the axis, asshown in Fig. 5. conside. elementary slice of the electric field const ing the lens. Let its thickness along the exis be Ax. When an electron passes through slice, its component of velocity 1j is 13;; anamount Ay, given by the equation where Ey is the electric force in the y direction and At is thev time required for the electron to travel through the element. InEqu-at'ion l elec- The plates lil, li of the first trlcal units are defined in a manner that eumlnates the ratio V of an electron. k

"I'he substitution Aof Y 4mf {The equationof the kinetic energy of the electronisn 'V where 1? is theelectric Vpotential .at lpointsfin Velectric iield. For beams of reasonable angu-V lar width. y issm'au compared witnei'andmay .5

be neglected in Equation 2.y This'gives 5 and ein Equation 1 gives Y Y v. roo

^ym'oyl o o jm Let :ci and @designate the substantial boundaries of the .electricileld constituting the lens.

. l 1 Elib Where the integral is to be taken along. the

Y particular electron path that iS'under considera-- tion, the values of and are the ones thaty occuzalongV -that particular path. Y As an approximation, variations in .y along an electron path through the lens can -be neg- .lected and y considered as constant along 'the 'patluthat is, the electron .is treated as if it `goes through the lens along apath paral/leito Vthe axis at a distance y, that substantially corre-- sponds to the actual path (see Fig. 6)

`The focal distance. d is? given by (see Fig. "7)Y in Equation 8 gives where the subscripts `again `refer to values at .1 r

:mand-:122.` Y

Ifa lens is to befree of 'aberrationfthe second focal distance da must be independent of the distance y at which an electron passes" through the lens,` otherwise `electrons vfrom Aa `common Vfocal point at rdistance vdi would come toa focus Sat this equation and VEquation 3 a different distancedaand the focus would ,Now @i and In `are theV diii'used alongthe axis. y Vpotentials ljust outside' or the lens, thatds, the potential airthez entrance side of the lens and thev -5 potential atfthe'exit side, 'and they are indep'end I --eritofgL' 'Ifhusif theflast term ofEquation 10 is independent` .of @y. theslens will be .substantially free f rorngaberration. This' demands that j 1 n i1'4 er I y El y Arf-constant I f" for, since :vis considered constant along nan elecwere vproportioiiai to il. and it isrnore likely to be truefift/li'at Afactor is roughly proportional to y. In any practical case the factor'cann'ot bemade exactlyY proportional to yibutit can. be-made of vthe departuresfirom proportionality cancel each other in thefintegral.V

occur whenfthe departures `fromj prolfiortionality To obtain a len'sjwith reduced aberrationf,l s av4 so-` lution of Equation- 1 3 f which also obeys r Equation 12 isneoessary. A furtherVV requirement is that the electron lens structurel correspond to twoVV sets oi parallel plates over most of their extent'l 40 `'I'he .aberrationof a lens is usually Vstated in termsoi' the aberrationratio.

focal distancef of rays rentering the 'Ihe focallength Zoran velectron path nearthe axis is designated as fo. The focal length for an electronpath near the edgeoi the lens is calcullated and designated as f; The aberration ratio at the edge oi the'lens lisi Y' For a'periect this ratio would be` equal to unity, and this' condition is yat 6 mately attainedinthestructure shown inFig. 2;!

has its edge facing the corres ponding plate inthe other "pair ilared. An abrupt are, suchas is 6 plates lin some tubes niay trodes overmost of i their Vvlength'. Moreover, for

.this requiresthe electric potential to be anfanalytic, function oi' Vthecoordinates a; and 'y, `The" electric .potential fr in such a' lens structure may be combined with 'its conjugate potentialirf to such afnaturethatit is" more than'V proportional in some regions-andless infothers an'd so that- ".This seems more likely to Itis based on the least approxi1V -In Fig. .12 4each of the Iplates i4, I Sand 16,11

showri-'in the drawings,'is required because the 'Y be us'ed' as sweep plates and :theysshould i',hereforevbe plane parallel elecdesign reasons, itis rdesirable to''calculateV the'AVV focal lengths-of the lens withoutlundue laborand give a Ycomplex potential function r-i-ifj on and it is seen that they where V1 is the potential of the first pair of plates Il, IEfand Va is the potential of the second pair ofplates IB, I1 and where S is the distance between the pairs oi plates Il, I and Il, I1 in the region where the plates are substantially parallel. The electric potential is The equation of the equipotential surface at Vi is sinh 2m:

cosh 2te-1- eos 2mg (m and the equation o! the equipotential surface at V2 iS These equipotential surfaces are shown in Fig. 2 correspond to two pairs of plates with dared ends. The shapes of the physical electrodes for the lens shown in Fig. 2 are consequently given by Equations 18 and 19.

It follows from Equations 18 and 19 that the maximum separation of a pair of plates occurs at x=0. The substitution o1 this value of a: in

The maximum separation is thus 2S, which is twice the minimumi separation S.

Italso follows from these equations that. at :c=S, the separation 2y is 1.035 which is only 3 per 'cent greater than the minimum separation S. Thus the ilare is substantially confined to a region along the axis equal to the minimum plate separation.

The

.by is obtained from Equation 16. The appreciable values of this derivative are confined to a narrow region, where the pairs of plates I4, I5 and I6, I1 iaceeach other, and covering a distance along the x axis about equal to S. In cases where the conjugate focal distances of the electron lens are large compared to S, it may be treated as a thin lens. Theinverse iocal term is then given approximately by the integral F y by 421 where y is considered al constant in the integration.

For paraxial electron paths--paths near the axis-cos 2y is approximately equal to unity, and sin 2y is approximately equal to 2y. With these approximations the integration for the inverse focal term gives The first principal focal distance f1 is -FV and mm Equation 21 1t may be written 1n the form where is the voltage ratio Va Vi The second principal focal distance I: is

f,= fn/E (23) The values of these principal focal distances f1 and l: are given as a flmction of voltage ratio in the curves shown in Fig. 4. These plotted values may be used in the lens equation ford calculating a coniugate focal distance di or z.

Equation 20 for the inverse focal term cannot in general be integrated for electron paths at a distance from the axis, but it can be integrated in series for the 'particular case of an electron path through the edge of the lens. that is, a path term of the resulting series constitutes a known deilnite integral, and the integration of the series gives path through the edge of the lens is -FV 5171 and it may be written in the form E -l L1) l [iufm (i+1 +1024 +1 L (29) The second principal iocal distance is again equal to From the preceding equations the aberration ratio f/fo is obtained at the edge of the lens; it is the ratio of a focal distance at the edge of the lens to the corresponding focal distance fo at the center of the lens. The inverse ratio .fo/f is, however, more descriptive in the present case, and this ratio is plotted as a function of voltage ratio in a curve shown in Fig. 4. This A' curve shows i that the correctionimproves they .isa positive grid element?! Ato'draw-secondari lens in adecided manner. It reduces the inverse electronsirom the split target and this is Vconaberration ratio from iniinity-Ior" ani,uncor nected to a 'point of thesaxn rected lens-to va uesnear the unity'ratio of a, 1 plates 36,'36 and the p1ates 33,f33. This potenperfect lens; Y 1 u The type, oi lens covered by this invention isV "potentiometerw which isY connected across a used asa component inthe two-lens combina suitable Vsource 4|'. {I 'n 'most negatvetapqz tion shownV in Flg.-3. l Such a combination com- OI the potentiometer,resistance 4U is v connected prises pairs of platesY I8; laand 20, 2| and 22, 23 to the plates 32, 32, Vthe Vnext tap i43-is connected .flared as inthe arrangement shown in Fig. 2, and 10 to theigcathode 3|, andthe tap44 is connected to spaced alongVv the axis ofthe tube.' V'I'othesepairsY the mid-point45of thesecondarywindingd of Vof, plates'fare'A applied lpotentials VV1'. Vi'and V1, fsa transfonnerllL'the primaryfwinding of respectively, V1 being more positive than Vaal- Which 1S connectedto a fsourcexof input signals vthough it is possible fort the potentials tobe vre- .-fOr Vdeflectinggthe sheet Yof electronsVv in 'the tube. verseaand Vita bemore'positivelthanV13;.The 15 lha-tap VUsi'cfinrlettectto the ridfpoint 50pof two lens systems v( thatis, one comprlsingrpavirs the primary winding 5|I of theV transformer 52 Y of. plates |8 ,-|9 and`202.|and the otherjcomthe secondary windina 0f which is inflectedY Vprising pairs of plates 20, 2| and 22, 23) tend vt0 all Outpublcircuit. I'h'eqterminalsofnthe prito correct each-other and any residual aberration mary Winding 5| 'are connected to the plates 38, inthe corrected lenses wlllbestill further reduced zo 39 forming thesplitgtarget- The intensityof Whe heyare used in this mannen The pair of the ilow of secondary electrons' from the target vcorrected lenses shown in Fig. 3 givesV a vclean?VV 3 8 39 t0 the'mesh grid 3Tdete1mi11s'the 011tj l put current of` the amplifier, The cathode 3| -Througho t descriptio'xfrtlfie term V,pairs of iS heated-by'lcurrent from a source lthrough Vbeenffusecifbutit is toffbeunderstood 2.3 resistance. 55!" j tht'me :arrangementoperatestjust as Wen (ex- `The operational the tube 30 is as follows: y

'cept whenthepair of plates. is used'fordeecting .The electron stream vfrom' the cathode 3| is Y,

'il-located inthe `flared regionsof plates 33, 33gand Y In Fig'.l ,4 focal distances in terms; of tube4 35 .fieectlngplates`-3QQ 3 5. The' Vsheetof electrons vwidth and inverse aberration ratio` are plotted` "1S deectef 'byisignals applied@ the sweenplats against voltage ratios., The principal focal dis- 34' 35 WhichY Signalsr may be Sine Waves,'saw tances f1 and fz areV shown vfor values of the Y tOOPhed Waves .01 Other-types 0f Complex Signal vvoltage ratio f 'e 7 Y Y a Y Y ,4o-electron lens La'whichls located in the VflaredVV VY2 1- Y i regions of the`-`sweep"plates 34,"35f`and' 36,; 36,

' The curves Vmax pavlsl be used 'fOr Voltage ratios 'motion ofi-this ima e across Vt 'li vr less than 1 byinterchangingVa and V1. and intel- 45 gives an amplified sgnalotpuina intnrgir' changing fl and fr at the same time. well known by'varyin'g! the potential]v of theplates The lens syste s'described above may -be usedfV -38 and'39 with respectto each other andV thus in Vany suitableelectron discharge device wherein Y settingy up Va'. Vcurrent Vin the itis desired to have two-dimensional electron lens `5| ofthe transformarl 52 whic' systems o f highquality'but they'are particularly 50 to the` input signal applied to the primary wincl- Y plate s. Suena y tubeis showninfie. 8.1 f substantially corrected` for aberration, thebeam In F1 g. 8 thereis disclosed a deflection ampllierV ,can substantially fill the region between the sweep Vtube with two-dimensional electron lenses cor-v 55 plates 34 and 3 5.v The sweep plates canthusV made in the `embodi. 30 enclosing a linear cathode 3| `to furnisha 6b ments disclosed above-without changing the scope stream of electrons, a pair ofplates 3|?,r 32 elecof the invention;Y as indicated 5 by the appended trically connectedtogether and placed at a negaclaims. f Y

- What is claimed tent andar Y y70 their extentto ariane' parallgljtqithe plates of at the saine potential as the plates ,33, 33, and

ysplit target38, 39 overwhich thefocussedribbonstream of electrons is Amoved 'by the deflecting' y pssesbetwent plateau; as, nl frontfof the split target 3s,i3s

`members, and means for Aparallel throughout most of their Iplate of the same pair,

in a portion thereof near the other pair of members, all means for maintaining at all times the potential oi one member of one of said pairs equal to that of the other memberof the same pair.

2. An electron `optical system comprising a pair of metallic plate members which are generally parallel throughout most of their extent, and a second pair or metallic plate members which are generally parallel throughout most of their extent and are also parallel throughout most of their extent to a plane parallel to the plates of the first pair throughout most o! their extent, said pairs of members being adjacent each other and having a common longitudinal axis which passeabetween` the metallic members of each pair and each lof said plates being :dared outwardly in a portion thereof near the other pair of members, the plates of at least one of said pairs of members constituting throughout most of their extent two of the four walls of an open box-like member having a rectangular cross-section, the entire box-like `member' being" at one potential.

3. An electron optical system comprising a pair oi? metallic plate members which are generally parallel throughout most oi their extent, a second pair of metallic plate members which are generally parallel throughout most of their extent and are also parallel throughout most of their extent to a plane parallel to the plates of the met `pair throughout most of their extent. said pairs ol members being adjacent each other andfhaving a common longitudinal axis which passes between the metallic members of eachpair and each of said `plates being iiared outwardly in a portion thereof near the other pair of members, the maximum distance `between the metallic members in a pair being substantially twice that of the minimum distance between said maintaining at all one member of one of said the other member of the times the potential of pairs `equal to that of same pair.

4. An electron optical system comprising a pair of metallic plate members which are generally extent, and a second pair oi.' metallic plate members which are generally parallel throughout most of their extent and are also parallel throughout most of their extent to a plane parallel to the plates of the ilrst pair throughout most of their extent, said pairs oi' members being adjacent each other and having a common longitudinal axis which passes between the metallic members of each pair and each of said plates being flared outwardly in a portion thereof near the other pair of members, `and means for maintaining one plate of each pair at the same potential as the other said potentials being different.

5. .An electron optical system comprising a, pair of metallic plate members which are generally parallel throughout most of their extent, a second pair of metallic plate members which are generally parallel throughout most of their extent, and a third pair of metallic plate members which are generally parallel throughout most oi their extent, the plates ol' the second and third pairs being also parallel throughout most of their extent to a plane parallel to the plates of the first pair throlmhout most of their extent, said pairs of members having a common longitudinal axis which passes between the metailicmembersfoi each pair and said pairs of members being spaced in the direction of said axis, each of the members of the first pair being ilared outwardly in portions thereof nearer the second pair of members; each of the members of the second pair being ilared outwardly in portions thereof nearer said rst pair of members and nearer said third pair of members and each of said third pair of members being ilared outwardly in portions thereof nearer the second pair of members. Y

6. An electron optical system comprising a pair of metallic plate members which are generally parallel throughout most of their extent, a second pair of metallic plate members which are generally parallel throughout most of their ex-` tent, and a third pair of metallic plate members which are generally parallel throughout most of their extent, the plates of the second and third pairs being also parallel throughout most of their extent to a plane parallel to the plates of the ilrst pair throughout most of their extent, said pairs of members having a common longitudinal axis which passes between the metallic members of each pair and said pairs of members b eing spaced in the direction of said axis, each of the members of the rst pair being flared outwardly in portions thereof nearer the second pair of members, each of the members of the second pair being flared outwardly in portions thereof nearer said first pair of members and nearer said third pair of members, and each of said third pair of members being flared outwardly in portions thereof nearer the second pair of members, the maximum distance between the metallic members in a pair being substantially twice that of lthe minimum distance between said members.

'1. An electron optical system comprising a pair of metallic plate members which are generally parallel throughout most oi their extent, a second pair of metallic plate members which are generally parallel throughout most of their extent, and a third pair of metallic plate memV bers which are generally parallel throughout most of their extent, the plates of the second and third pairs being also parallel throughout most of their extent to a plane parallel to the plates of the first pair throughout most of their extent,

` said pairs of members having a common longitudinal axis which passes between the metallic members of each pair and said pairs of members being spaced in the direction of said axis, each of the members of the first pair being flared out,- wardly in portions thereof nearer the second pair of members, `each ofthe members ofthe second pair being flared outwardly in portions thereof nearer said rst pair of members and nearer said third pair of members, and each of said third pair of members being flared outwardly in portions thereof nearer the second pair of members, means for maintaining the first pair of members and the third pair of members at substantially the same average potential, and means for maintaining the second pair of members at an average potential which is dierent from that of the first and third pairs of members.

8. An electron optical system comprising a pair of metallic plate members which are generally parallel throughout most of their extent, a second pair of metallic plate members which are generally parallel thigughout most of their extent and are also parallel throughout most of their extentto a plane parallel to the plates of the first pair throughout most of their extent,

of said deflecting plates which are adjacent said other pair'of plates similarly curved outwardly,v

sheet a deection at the target which is small' compared with the thickness of said sheet between the deiiecting plates.

18. An electron optical system comprising two pairs of metallic members, each member having a substantially plane portion and all said plane portions being parallel to a plane between the members of a pair, each of said members having an end portion ilared outwardly in a direction away from said plane with its extremity spaced a short distance from that of the flared end portion of a member of the other pair, and means for applying a constant potential to one of said pairs of members and a varying detlecting potential between themembers of the other of said` pairs.

19. An electron lens comprising two pairs of metallic members, each member having a substantially plane portion and all said plane portions being parallel to a plane between the members of a pair, each of said members having an outwardly flared portion adjacent; the correspending member` of the other pair which ared portion presents a convex surface portion to said corresponding member and has its extremity spaced a short distance from that of the iiared end portion of a member of the other pair, and means for maintaining at all times the potential of one member of one of said pairs equal to that of the other member of the same pair.

20. 'Ihe combination of elements as in claim 18 in further combination with means for maintaining at all times the average of the potentials i of the members of the pair to which the varying deiiecting potential is applied different from the constant potential applied to the members of the other pair.

21. The combination of elements as in claim 19 in further combination with means for maintaining at all times the potential of one member of the other of said pairs equal to that of the other member of that pair which potential is different from that of the potential of the members of said one of said pairs.

FRANK GRAY. 

